Gas-rich and intensely luminous, distant submillimeter galaxies (so termed for their emission wavelength, not their spatial extent) form stars at very high rates. It has been suggested that interactions between the galaxies power the emission, but most studies so far have been marred by the presence of dust, which can obscure or produce misleading signs of interaction. To circumvent this problem, Engel et al. analyzed high-resolution observations of CO emission lines from 12 submillimeter galaxies, 4 of which were newly observed. The CO lines trace the molecular gas that fuels star formation, and their observation is not affected by the presence of dust. Of the 12 galaxies analyzed, 5 were double systems, consisting of two galaxies engaged in the early stages of interaction; the remaining 7 either showed signs of being at an intermediate stage of merging or were compact galaxies, plausibly the end result of two galaxies coalescing into one. These results indicate that most submillimeter galaxies, if not all, are the product of interactions and mergers of galaxies.